CS121: Our Digital World

Transcription

1 + CS121: Our Digital World

2 + Redacted Documents 2

3 + Activity: Badly Redacted 3 Documents Download the following file: Can you find the secret message?

4 + Electronically Redacted Document 4 Examples TSA: Screening Management Standard Operating Procedure Two examples: 2009 and 2010 US Military Washington Post What went wrong? WYSIWYG

5 + Number Systems ASCII, binary, decimal, hex 5

6 + ASCII 6 In text files, each character is represented as an ASCII character ASCII Table: So, the letter a is represented as: Decimal: 097 Binary: Hex: 061 A is represented as: Decimal: 065 Binary: Hex: 041

7 + Decimal Numbers 7 Start counting: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 Oops! We ve ran out of digits Add a second column: 10, 11, 12,., 19, 20, 21,, 98, 99 Oops! We ve ran out of digits Add a third column and so on

8 + Decimal Numbers 8 Another way: 365 is: 3 x x x 1 Or 3 x x x 10 0

9 + Binary Numbers 9 Start counting 0, 1 Oops, we ve run out of digits Add a column: 00, 01, 10, 11 Oops, we ve run out of digits Add another column: 000, 001, 010, 011, 100, 101, 110, 111 And so on.

10 + Binary Numbers 10 Another way: 1010 is: 0 x x x x 2 0

11 + Activity: Binary Numbers 11 Convert these numbers from binary to decimal Represent the following decimals in binary

12 + Track Changes & Metadata 12

13 + Changes that can be Traced 13 Track Changes Metadata Which also can be forged

14 + Track Changes Embarrassment 14 UN document on the assassination of Lebanese Prime Minister Rafiq hariri Dodgy Dossier used by British government to justify their 2003 entry into the war on Iraq

15 + Representation 15 ASCII vs. raster What are the pros and cons of each? Example: Book of Kells

16 + Compression 16 Compression is necessary sometimes due to the size of the original image or file Two kinds of compression: Lossless Lossy Raster example:

17 + Hiding Things 17

18 + Photo Editing 18 Microsoft edited a photo advertisement

19 + Steganography 19 It is the art and science of writing a message in a way to where the only the recipient knows of its existence. The word Steganography is of Greek origin and means covered, or hidden writing. Steganographic messages will generally appear as something else such as a picture or a text file. Provide security over insecure channel

20 + Steganography 20

21 + Watermarked pdf Example 21

22 + Activity: Watermarked PDFs 22 Download the pdf file to the desktop Open it and view it in Preview. Open Terminal and type the following: cd Desktop head -2 mad-men-denouement.pdf What did you notice?

23 + Watermarked PDFs 23 Download the python script ( ) Type the following in Terminal: python sign-pdf.py mad-men-denouement.pdf JaneDoe ls *.pdf tail -2 JaneDoe.pdf Repeat the process above using a different name

24 + Watermarked PDFs 24 Okay, the files look identical, but can we tell the difference at the bit level? Yes. There is a way of boiling down a file (of any length) down to a 128 bit signature. The technique is called MD5 (the MD stands for Message Digest ). The digest goes by several names, such as checksum (since it's very often used to check the integrity of a file: is a copy correct?). Download the python file ( ) then type in the following in the terminal: python md5.py *.pdf Notice that each file has been boiled down to a signature or checksum. In this case, it's 128 bits long, and the bits are described by a string of hexadecimal digits. This allows us to detect any tampering with the file, as well as being able to ascertain identity.

25 + Activity: Hard Disks 25 How do they work? How can hard disks be truly sanitized? Conduct a little research and come up with three techniques:

26 + References /12/the-tsa-makes-another-stupid-move/ publicexamplesoftrackchanges.html topic=core-pre-algebra